This invention relates to a heat shrinkable synthetic resin cover containing a heat generating element and adapted for use as an anticorrosion coating member applied to conduit joints, and a protective coating member for the joints of electric wires and cables.
Heat shrinkable synthetic resin covers have been widely used for protection against corrosion and mechanical impact of joints of natural gas or petroleum pipelines or joints of wires or cables. For example, after being applied about a joint of steel pipes or electric wires or cables, a sheet or tube-shaped heat shrinkable synthetic resin cover is heated from outside with the flame of a gas burner to shrink so as to fit and adhere tightly to the joint.
Examples of such heat shrinkable covers used at such joints are described in Japanese Preliminary publication of patent No. 119,684 of 1977 (corresponding to U.S. Patent Application 5 filed on March 5, 1976, now abandoned) and U.S. Pat. No. 3,086,242.
According to the prior art of coating a joint, when a heat shrinkable synthetic resin cover is applied about the joint, a suitable external source of heat is required to shrink the cover. Unless the heat shrinkable cover so applied is uniformly heated about the entire periphery, there are such problems as non-uniform shrinkage causing air voids and uneven thickness of the cover. Although such coating operations are generally performed in the field, use of a heating furnace is not suitable for the field work so that in most cases gas burners have been used. With the gas burner it is generally difficult to heat uniformly all the periphery at the same time.
For example, when a heat shrinkable cover is wrapped about a joint of pipes having an outer diameter larger than one meter, the area of the cover heated at a time by a LPG gas burner is relatively narrow so that, it is impossible to uniformly heat the entire surface of the cover. In order to uniformly heat the entire surface, it is necessary to use a special multiple head burner or to simultaneously perform the heating operation with many skilled operators.
Where the field is in extremely cold areas or windy areas, it has been impossible to uniformly and simultaneously heat the entire surface of the heat shrinkable cover with external heating means. Nonuniform heating results in destruction of the cover due to overheating or an insufficient bonding between the joint and the cover thus forming an air gap therebetween or nonuniform wall thickness of the cover. Thus, as it is impossible to maintain the corrosion proof property at the joints over a long period, and since nonuniform heating results in nonuniform residual stress in the coating the durability and weather proofness of the coated layer degrade thus making it impossible to use it over a long period.
In order to solve the problem as described above, the inventors have proposed one solution in the U.S. patent application with a Ser. No. 68,761 entitled "Heat Shrinkable Covers", which was filed on Aug. 23, 1979 claiming the Convention priority from the Japanese patent applications No. 53-106199 and No. 53-106200 filed on Sept. 1, 1978.
According to said prior invention, there is provided a heat shrinkable synthetic resin cover of such a type as sheet or tube which comprises a heat shrinkable synthetic resin member, and a flexible electric heating member coated with a cross-linked polymer is embedded in the heat shrinkable synthetic resin member so that the electric heating member does not preclude heat shrinkage of the heat shrinkable synthetic resin member in a direction inherent thereto. Further, the heat shrinkable rectangular synthetic resin member used in this invention is made of a polymer, and an electric heating wire is distributed therein in a zig-zag configuration so that it crosses substantially at right angles to an inherent direction of heat shrink of heat shrinkable synthetic resin member.
The present invention, therefore, aims at the improvement of the prior invention that the inventors proposed in the past.